Biopolymer hydrolysis and bacterial production under ambient hydrostatic pressure through a 2000 m water column in the NW Mediterranean

Kinetic parameters for aminopeptidase, phosphatase, and bacterial production rates were studied during spring and fall through a 2000 m water column in the NW Mediterranean. Bacterial production ranged from 60.4 ng at 30 m to 0.2 ng C l−1 h−1 at 2000 m. For both ectoenzymatic activities, the Km values ranged from 0.44 to 1.13 μM for aminopeptidase activity and from 0.05 to 1.23 μM for phosphatase activity. Depth profiles of the potential activity of aminopeptidase and phosphatase activity drastically decreased below depths of 100 m. At 1000 m, hydrolytic activities were one order of magnitude lower than the maximal rate measured in the surface layer. Despite this decrease, depth-integrated rates through the thickness of different water masses showed that the potential hydrolysis fluxes within the productive surface layer (10–200 m), through the twilight zone (200–1000 m depth), and through the deep water mass (1000–2000 m) were roughly the same order of magnitude. This study used the first assay for measuring ectoenzymatic activities of deep-sea microbial populations where pressure stresses were eliminated during sampling and incubation. The results showed that prokaryotic induced ectoenzymatic activities are affected by pressure conditions. Generally, aminopeptidase and phosphatase rates measured in samples maintained under in situ pressure conditions were 2.3 times higher than those measured in their decompressed counterparts.